It is known that when the dew point of an atmosphere gas in a continuous annealing furnace is −45° C. or less, surface segregation of Mn during annealing can be suppressed, and adhesion of zinc or zinc alloy plating after annealing is improved (see Tetsu To Hagane (Bulletin of the Iron and Steel Institute of Japan), 96-1 (2010), pp. 11-20).
The following are examples of a method in the related art to reduce the dew point of an atmosphere gas in a continuous annealing furnace.
A: A method of supplying another atmosphere gas having a low dew point from the outside of a furnace to a heating zone or a soaking zone (see Japanese Unexamined Patent Application Publication No. 2002-3953).
B: A method of providing a mechanism to circulate a furnace atmosphere gas in the outside of the furnace and thereby performing heat exchange between the circulating high-temperature atmosphere gas and a room-temperature atmosphere gas having a low dew point, which is to be supplied separately to the furnace (see Japanese Unexamined Patent Application Publication No. 62-290830).
C: A method of performing heat exchange between a high-temperature furnace atmosphere gas and an atmosphere gas having a dew point that has been reduced in the outside of a furnace and reducing the dew point with a water adsorption filter (see Japanese Unexamined Patent Application Publication No. 11-124622).
In accordance with method A, the low-temperature gas is directly introduced into the high-temperature furnace. Thus, a large amount of thermal energy is required to maintain the steel strip temperature in the furnace, the gas temperature cannot be controlled, and the energy efficiency is very low.
In accordance with method B, even when the low-temperature gas has a low dew point, the low-temperature gas is mixed with a large amount of atmosphere gas having a high dew point in the furnace. Thus, the dew point of the atmosphere gas in the furnace cannot be sufficiently reduced.
In accordance with method C, as described in Japanese Unexamined Patent Application Publication No. 11-124622, the dew point is reduced to at most −30° C. using the water adsorption filter having a low dehumidification capacity. Thus, a very low dew point (−45° C. or less) of the atmosphere gas cannot be achieved. Furthermore, the energy efficiency is low. Thus, known techniques to reduce the dew point of the atmosphere of a continuous annealing furnace have problems that they cannot achieve a low dew point of −45° C. or less and that they have very low energy efficiency.